Spray drying is a drying process, which involves both particle formation and drying. It involves atomisation of a feed, typically a liquid concentrate, into a spray and contact between the spray and a drying medium. The formation of the spray (atomisation) and the contacting of the spray with the drying medium may be achieved by use of a nozzle.
Pneumatic nozzle atomisation involves impacting a liquid feed with a high velocity gas. The high velocity gas creates high frictional forces and disintegrates the liquid feed into spray droplets. The feed liquid is believed to break-up in two stages. The first phase involves the tearing of the liquid feed into filaments and large droplets. The second phase completes the atomisation by breaking these liquid forms into smaller and smaller droplets. The entire process is influenced by the magnitude of the surface tension, density, pressure and viscosity of the liquid feed as well as the velocity and density of the gaseous flow.
Various design techniques are available to produce the required conditions of liquid-gas contact for atomisation. As disclosed in the book “Spray drying” by Keith Masters, 1991 edition, page 251, the designs may be divided into 4 categories:    (1) Internal mixing in which liquid feed and atomising gas are contacted within the nozzle head.    (2) External mixing, in which liquid feed and atomising gas are contacting outside the nozzle head.    (3) Combined internal and external mixing by using two atomising gas flows within the nozzle head (three-fluid nozzle).    (4) Pneumatic cup atomising, in which feed liquid and atomising gas is contacted at the rim of a rotating nozzle head.
The different design techniques provide different properties and result in different outcome of the final atomised product. In the first 2 categories the feed liquid and atomising gas are passed separately to the nozzle. Such nozzles, which are usually denoted two-fluid nozzles (TFN), are i.a. used for atomisation of a liquid in spray drying plants and in fluid bed agglomeration. The liquids can be in the form of solutions, dispersion or pure substances. In particular, two-fluid nozzles are used when atomising a fluid, where fine droplets is the objective or where additional atomisation energy in the form of atomising gas is required to break up a fluid into droplets. Nozzle designs of the third and fourth category are not the subject of the present application.
Internal mixing TFN has the advantage, compared to external mixing TFN, that it is mixing gas and liquid before the two fluids enter the surrounding atmosphere of the drying chamber. However, nozzles providing internal mixing are not as well suited for handling abrasive feeds as the internal mixing introduces additional wearing of the equipment. Conventional two-fluid nozzles with internal gas/liquid mixing also introduce the risk of drying out and thereby clogging the mixing chamber.
Internal mixing nozzles give the possibility of an efficient liquid-gas reaction, but are limited in capacity by internal channelling and channel dimensions. Internal parts in the nozzle, intended for improving the gas-liquid mixing, also disturb the flow, causing the span of the droplet size distribution to rise. Internal parts in general complicate handling, cleaning and causes wear. Furthermore viscous liquid feeds may be difficult to process.
Examples of nozzles of the internal mixing type are well known in the art. U.S. Pat. No. 7,694,944 (GEA Niro) discloses a nozzle in which the gas is supplied in the axial direction of the nozzle. The nozzle comprises a mixing chamber, one or more liquid inlets and at least one tangential gas inlet to the mixing chamber. In a commercially available internal mixing nozzle the atomising gas is supplied tangentially in a separate pipe, which contributes to the radial dimensions of the nozzle. Furthermore, the mixing chamber of this prior nozzle comprises edges and obstructions resulting from structural conditions. International published application WO 00/58014 discloses a sprayer in the form of a nozzle having a tangential gas inlet to the mixing chamber and lateral liquid inlets. This nozzle suffers from insufficient mixing due to the geometry of the nozzle.
Criterions for evaluating the performance of a two-fluid nozzle are: the mean droplet size, the span of the droplet size distribution and not least the specific gas consumption, meaning the amount of gas used to atomise a given amount of liquid, also called the gas-to-feed ratio. In addition to the criterions focused on the product quality, the production capacity of the two-fluid nozzle is also of high importance—especially from a commercial point of view. Furthermore, increasing focus on clean technologies as well as increasing energy prices put forward additional requirements with respect to energy consumption when operating and producing by spray processes.
The contact and mixing of gas and liquid is where external mixing TFN meet their restrictions. External mixing TFN, where the gas mixes with the liquid after leaving the nozzle typically through a ring-shaped aperture, meets the limitation when the gap in the gas exit becomes so large that a larger part of the gas is lost into the surrounding atmosphere of the drying chamber, instead of reacting with the liquid. With external mixing TFN, the free expansion of the gas has the disadvantage of being partly lost to the surrounding instead of adding energy to break up the liquid. In the prior art this problem has been attended.
Another type of nozzles utilizes pressurization of the liquid, meaning that the feed concentrate is fed under pressure to the nozzle. Pressure energy is converted to kinetic energy, and feed issued from the nozzle orifice as a high speed film that readily disintegrates into a spray as the film is unstable. Sprays from pressure nozzles handling high feed rates are generally less homogeneous and coarser.
EP 408 801 B1 suggests a low pressurized liquid, internal mixing two-fluid nozzle which can function satisfactory even when low pressure is applied during a period of start-up as small droplets are produced. The spray nozzle unit is provided with a gas slit between the pressure nozzle and the air nozzle to give a part of the discharging air stream a swirling motion.
The present invention is directed to a high pressurized liquid external mixing two-fluid nozzle that efficiently uses the atomizing gas. It is well known within the art that the disadvantages of pneumatic nozzles concern the high cost of compressed air and low nozzle efficiency. Furthermore, a drawback with several of the pre-existing, conventional two-fluid spray nozzle units is the limited capacity when very fine droplets are required. The object of the present invention is to provide for an external mixing pressurized two-fluid nozzle, which is energy efficient, provides high capacity while still producing fine droplets.